1. Field of the Invention
The present invention relates to a loudspeaker. More specifically, the invention relates to a loudspeaker having an elongate structure including a diaphragm of a small width.
2. Description of the Related Art
Conventionally, it is most common to form loudspeakers in a round shape. However, in recent years, loudspeakers having an elongate structure have increasingly been demanded. Such loudspeakers having an elongate structure are widely used for television sets, for example. Lately, the sound to be reproduced by loudspeakers provided for a television set is frequently received in stereophonic sound, not in monaural sound. Accordingly, in many cases, loudspeakers to be provided for such a television set are now disposed on right and left sides of the Braun tube thereof. In such a case, it is preferable to provide loudspeakers of a small width, i.e., having an elongate structure (hereinafter, referred to as "slim loudspeakers") for a television set so that the lateral width of the television set becomes as small as possible.
The voice coil bobbin of a conventional loudspeaker having an elongate structure is generally of a round shape. Such a voice coil bobbin in a round shape is attached to the center portion of an elliptical, oval or oblong cone-shaped diaphragm so as to drive the cone-shaped diaphragm. The voice coil bobbin is further supported by a round or elliptical damper (hereinafter, a loudspeaker having such a structure will be called "a cone-shaped slim loudspeaker"). A cone-shaped slim loudspeaker of this type has the following problems.
In general, it is difficult for a cone-shaped slim loudspeaker to reproduce sound in a low frequency band because of the following reasons. In a cone-shaped slim loudspeaker, the diameter of a damper is required to be small. If the diameter of a damper is small, then the rigidity thereof becomes large. As a result, the lowest resonance frequency f.sub.0 of a loudspeaker becomes high, and therefore the frequency characteristics in a bass sound region are degraded.
A large input can not be applied to a cone-shaped slim loudspeaker. In general, as an input to be applied to a loudspeaker becomes larger, the amplitude of a diaphragm also becomes larger. Since the diameter of a damper of a cone-shaped slim loudspeaker is much smaller than the larger diameter of a diaphragm, a rolling is likely to occur particularly about a larger diameter direction when the amplitude of a diaphragm is large. In the case where a diaphragm rolls in the larger diameter direction, the voice coil sometimes comes into contact with a magnetic circuit depending on the degree of the rolling. Such contact causes abnormal sound, and, in some cases, damages the loudspeaker.
In a cone-shaped slim loudspeaker, large peaks and dips are generated in the relationship between the frequency and the reproduced sound pressure level. Such phenomenon causes undesirable sound quality. The peaks and dips are generated because a higher harmonic resonance is more likely to occur in the larger diameter direction in an elliptical or oval cone-shaped diaphragm as compared with a round diaphragm. The generation of the "higher harmonic resonance" means that the nodes of the vibration of a diaphragm exist at the positions other than the peripheral portion of the diaphragm, that is to say, the resonance is generated in a plurality of regions of a single diaphragm. Accordingly, the resonance frequency when the higher harmonic resonance is generated is higher than the resonance frequency when the higher harmonic resonance is not generated.
The reproduction frequency bandwidth tends to be small in a cone-shaped slim loudspeaker, so that the reproduced sound quality, i.e., the frequency characteristic when the sound is reproduced, becomes degraded. In general, in a cone-shaped loudspeaker, if the reproduction frequency becomes higher than a frequency f.sub.h at a predetermined level, the driving force by a voice coil bobbin is no longer transmitted to the entire portion of the cone-shaped diaphragm. As a result, the reproduced sound pressure level is drastically reduced. The larger the ratio of a larger diameter to a smaller diameter (hereinafter, simply referred to as a "larger/smaller diameter ratio") of a diaphragm of a loudspeaker becomes, the lower this frequency f.sub.h becomes. Since the diaphragm of a cone-shaped slim loudspeaker has a large larger/smaller diameter ratio, the frequency f.sub.h is low. In other words, the reproduction characteristics in a high frequency band are not satisfactory, so that the reproduction frequency bandwidth thereof becomes small.
On the other hand, a dome-shaped loudspeaker, i.e., a loudspeaker having a different structure from that of the above-described loudspeaker, is described in U.S. Pat. No. 3,935,400, for example. As disclosed in the patent, such a dome-shaped loudspeaker has an advantage in that the loudspeaker may improve the frequency characteristics up to a high sound region. However, the loudspeaker has the following problems.
A large input cannot be applied to a dome-shaped loudspeaker in a low sound region where the amplitude of a diaphragm becomes large. Since the dome-shaped loudspeaker is supported only by an edge portion, a rolling is likely to occur particularly about a larger diameter direction when the amplitude of a diaphragm becomes large. As a result, the voice coil may possibly come into contact with a magnetic circuit.
In the case where the larger/smaller diameter ratio of the diaphragm is set to be large while still using the structure of a dome-shaped loudspeaker, parallel linear portions of a voice coil become longer. In such a case, at certain frequencies (or resonance frequencies), resonance is generated in the linear portions of the voice coil bobbin, so that the linear portions vibrate in a direction vertical to the vibration direction of a diaphragm (i.e., the direction of magnetic fluxes within an air gap of a magnetic circuit for driving the voice coil bobbin). The longer the linear portions become, the lower the resonance frequency becomes. The amplitude of the resonance increases as the resonance frequency becomes lower. Accordingly, as the larger/smaller diameter ratio of the diaphragm becomes larger, the resonance amplitude of the voice coil bobbin becomes larger. Consequently, if a larger input is applied to a loudspeaker, this resonance amplitude also becomes larger, so that the voice coil may possibly come into contact with the magnetic circuit.